MTA station cleaner Sean Monroe saw it all unfold. He was cleaning the corridor at the moment the bomb went off nearby.

"Supervisor told me, 'Hey, there's food to clean down there at the end of the station,'" Monroe recalled. "So I'm like, 'All right, no problem.' I got my broom and dustpan in hand."

Seconds later, he saw the man, identified by authorities as Akayed Ullah, 27, try to blow himself up.

"I just saw the explosion. Guy just, 'boom!'" Monroe said. "People around him fell to the floor. As soon as they fell, they got right up and started running."

Monroe described a scene of panic as frightened straphangers ran for their lives:

"You panic for a second, but when you see all those people getting up, rushing, your first instinct is — especially with your training from MTA — first thing you think is, 'Let me try to direct these people out of here,'" Monroe said.

That kind of assistance, according to MTA Chairman Joe Lhota, is how police and transit officials had mapped it out during a drill last month.

"We had a tabletop exercise with the NYPD to coordinate our efforts in the events something like this ever happened," Lhota said at press conference after the explosion.

But the failed attack does highlight the vulnerability of a subway system with 472 stations and close to 6 million riders every weekday.

Tony Utano, the president of Transport Workers Union Local 100, said the would-be suicide attack points to the need for more security throughout the subway system — something that the union has been calling for.

"Our members cannot act like police officers," Utano said. "When a situation like this happens, they'll do the best they can to get people out of there. But if police officers are in the stations, there will be maybe less of these instances."

But at least this time, there was a quick-thinking station cleaner to help people to safety.